The present invention relates to metal alloys, and in particular, to a method for revealing deformed and recrystallized structures in single phase alloys.
Metallographic techniques are widely used in research and in industry. Typical uses include investigation of the phase constitution of metallic systems, the microstructure of metals, phase transformations, plastic deformation mechanisms, production heat treatment control and the analysis of the causes of service failures of metallic objects.
The selection and preparation of representative samples for examination is essential. Typically, a specimen is first ground and polished using decreasing grit sizes. These operations render the specimen surface scratch-free and mirrorlike by means of the progressive removal of surface irregularities. Alternatively, the specimen may be electrolytically polished by controlled anodic dissolution.
Polished specimens reveal only a few structural features, such as inclusions, microcracks and microporosity. Etching with an appropriate etchant is necessary to bring out the microstructures. Mechanically polished specimens have a thin surface layer of distorted metal; etching removes the distorted layer.
Etchants include acids, bases and complex substances, generally in dilute solution in water, alcohol, or glycerin. Most reagents act by dissolution, but a few act by selective deposition of reaction products. In a single-phase alloy, etching attacks different parts of the structure selectively. Grain-boundary regions dissolve in preference to the body of the grains and the resulting grooves appear as a dark network. In multiphase alloys the phases are attacked selectively.
Etching by heating the sample in a high vacuum or an oxidizing atmosphere has also proven useful. Thermal etching produces a groove in the metal surface where it is intersected by a grain boundary. Selective evaporation along crystallographic steps is also evident.
We have discovered that chemical and thermal etching may be successively combined to provide a new analytical tool.
Accordingly, it is an object of the present invention to provide a novel method for revealing the microstructure of metal alloys.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention.